Optical expander device for providing an extended field of view

ABSTRACT

An optical device comprises a waveguide plate, which in turn comprises: an in-coupling element to form first guided light and second guided light by diffracting input light, first expander element to form third guided light by diffracting the first guided light, second expander element to form fourth guided light by diffracting the second guided light, and an out-coupling element to form first output light by diffracting the third guided light, and to form second output light by diffracting the fourth guided light, wherein the out-coupling element is arranged to form combined output light by combining the first output light with the second output light, wherein the in-coupling element has a first grating period for forming the first guided light, and wherein the in-coupling element has a second different grating period for forming the second guided light.

CROSS REFERENCE TO RELATED APPLICATIONS

The present application claims the benefit of Chinese Patent ApplicationNo. 202110006780.0 filed on Jan. 5, 2021. All the above are herebyincorporated by reference. the contents of which are incorporated hereinby reference in their entirety.

TECHNICAL FIELD

The present invention relates to an optical diffractive beam expanderfor use, e.g., in a virtual display device.

BACKGROUND TECHNOLOGY

Referring to FIG. 1 , an expander device EPE0 comprises a waveguideplate SUB01, which in turn comprises a diffractive in-coupling elementDOE01, a diffractive expander element DOE02, and a diffractiveout-coupling element DOE03. The expander device EPE0 forms an outputlight beam OUT1 by diffractively expanding light of an input light beamIN1.

The input light beam IN1 may be generated by an optical engine ENG1. Theoptical engine ENG1 may comprise e.g. a micro display DISP1 andcollimating optics LNS1.

The in-coupling element DOE01 forms first guided light B1 by diffractinginput light B1. The expander element DOE02 forms expanded guided lightB3 by diffracting the first guided light B1. The out-coupling elementDOE03 forms output light OUT1 by diffracting the expanded guided lightB3.

The expander device EPE0 may expand a light beam in two transversedirections, in the direction SX and in the direction SY. The width wOUT1of the output light beam OUT1 is greater than the width wIN1 of theinput light beam IN1. The expander device EPE0 may be arranged to expanda viewing pupil of a virtual display device, so as to facilitatepositioning of an eye EYE1 with respect to the virtual display device. Ahuman observer may see a displayed virtual image in a situation wherethe output light is arranged to impinge on an eye EYE1 of the humanviewer. The output light may comprise one or more output light beams,wherein each output beam OUT1 may correspond to a different image pointof a displayed virtual image VIMG1. The expander device may also becalled e.g. as an exit pupil extender.

The displayed virtual image VIMG1 may have an angular width LIM1. Anattempt to use the expander device EPE0 of FIG. 1 for displaying amulti-color virtual image VIMG1 may cause a situation where red or bluelight corresponding to a corner point of the virtual image VIMG1 doesnot fulfill the criterion of total internal reflection when propagatingin the waveguide plate SUB01. Consequently, one or more corner regionsof the displayed multi-color virtual image VIMG1 may exhibit a lack ofred or blue color.

SUMMARY

An object is to provide an expander device. An object is to provide amethod for expanding a light beam. An object is to provide a displaydevice. An object is to provide a method for displaying an image. Theexpander device may be arranged to provide an extended field of view.

According to an aspect, there is provided an optical device (EPE1)comprising:

-   -   a waveguide plate (SUB1) comprising:    -   an in-coupling element (DOE1) to form first guided light (B1 a)        and second guided light (B1 b) by diffracting input light (IN1),    -   first expander element (DOE2 a) to form third guided light (B2        a) by diffracting the first guided light (B1 a),    -   second expander element (DOE2 b) to form fourth guided light (B2        b) by diffracting the second guided light (B1 b), and    -   an out-coupling element (DOE3) to form first output light (OB3        a) by diffracting the third guided light (B2 a), and to form        second output light (OB3 b) by diffracting the fourth guided        light (B2 b),    -   wherein the out-coupling element (DOE3) is arranged to form        combined output light (OUT1) by combining the first output light        (OB3 a) with the second output light (OB3 b), wherein the        in-coupling element (DOE1) has a first grating period (d_(1a))        for forming the first guided light (B1 a), and wherein the        in-coupling element (DOE1) has a second different grating period        (d_(1b)) for forming the second guided light (B1 b).

Other embodiments are defined in the claims.

The scope of protection sought for various embodiments of the inventionis set out by the independent claims. The embodiments, if any, describedin this specification that do not fall under the scope of theindependent claims are to be interpreted as examples useful forunderstanding various embodiments of the invention.

The expander device may be arranged to display a multi-color image,wherein the multi-color image may have an extended width. The multicolor image may be e.g. an RGB image, which comprises red (R) light,green (G) light, and blue (B) light.

Increasing the width of the displayed image may cause leakage of bluelight and/or red light of the corner points of the displayed image. Inother words, the in-coupling element of the expander device may form redlight or blue light, which cannot be confined to the waveguiding plateby total internal reflection.

The expander device may be arranged to provide two different routes forlight, in order to overcome limitations set by the capability of thewaveguiding plate to guide light of different colors in directions,which correspond to a wide image.

The expander device may split input light to propagate to theout-coupling element via a first route and via a second route. The firstroute may pass from the in-coupling element to the out-coupling elementvia the first expander element. The second route may pass from thein-coupling element to the out-coupling element via the second expanderelement. The first route may be optimized e.g. for guiding blue light ofa corner point, and a second route may be optimized e.g. for guiding redlight of the corner point. Consequently, the expander device may bearranged to display all corner points of an image in red and blue color.Red light of a corner point may be guided via at least one route, andblue light of the corner point may be guided via at least one route.

As a consequence of the optimization and the extended angular width ofthe displayed image, the first route may exhibit loss of red light of acorner point of the displayed image. The second route may exhibit lossof blue light of a corner point, respectively. However, red lightpropagating along the second route may at least partly compensate lossof red light from the first route. Blue light propagating along thefirst route may at least partly compensate loss of blue light from thesecond route.

The in-coupling element may comprise first diffractive features todiffract light to the first expander element. The in-coupling elementmay comprise second diffractive features to diffract light to the secondexpander element. The first diffractive features may have a firstgrating period and the second diffractive features may have a seconddifferent grating period. The first grating period may be selected toensure that blue guided light of a corner point is confined to thewaveguiding plate. The second grating period may be selected to ensurethat red guided light of the corner point is confined to the waveguidingplate. The first diffractive features may have a first orientation andthe second diffractive features may have a second different orientation.

The two routes may together at least partly compensate an error of thecolor of the corner point of the displayed image. The two routes mayreduce or avoid an error of the color of the corner point of a widedisplayed multi-color image. The two routes may improve uniformity ofcolor of a wide displayed multi-color image.

The out-coupling element may form first out-coupled light by diffractingthe third guided light, which propagates along the first route. Thethird guided light may be received from the first expander element. Theout-coupling element may form second out-coupled light by diffractingthe fourth guided light, which propagates along the second route. Thefourth guided light may be received from the second expander element.The first output light may spatially overlap the second output light.The out-coupling element may form combined output light by combining thefirst output light with the second output light.

The out-coupling element may comprise first diffractive features todiffract guided light received from the first expander element. Theout-coupling element may comprise second diffractive features todiffract guided light received from the second expander element. Thefirst diffractive features may have a first grating period and thesecond diffractive features may have a second different grating period.The first grating period may be selected to ensure that blue guidedlight of a corner point is confined to the waveguiding plate. The secondgrating period may be selected to ensure that red guided light of thecorner point is confined to the waveguiding plate. The first diffractivefeatures may have a first orientation and the second diffractivefeatures may have a second different orientation. The first diffractivefeatures may have low or negligible efficiency for coupling lightreceived from the second expander element out of the waveguiding plate.The second diffractive features may have low or negligible efficiencyfor coupling light received from the first expander element out of thewaveguiding plate.

BRIEF DESCRIPTION OF THE DRAWINGS

In the following examples, several variations will be described in moredetail with reference to the appended drawings, in which

FIG. 1 shows, as a comparative example, an expander device,

FIGS. 2 a to 2 e show, by way of example, in a three-dimensional view,forming input light beams by using an optical engine,

FIG. 2 f shows, by way of example, in a three-dimensional view, viewinga displayed virtual image,

FIG. 2 g shows, by way of example, angular width of the displayedvirtual image,

FIG. 2 h shows, by way of example, angular height of the displayedvirtual image,

FIG. 3 a shows, by way of example, in a front view, an expander device,which provides two different routes for in-coupled light,

FIG. 3 b shows, by way of example, in a front view, out-coupling regionsof the out-coupling element,

FIG. 4 a shows, by way of example, in a three-dimensional view, adisplay apparatus, which comprises the expander device,

FIG. 4 b shows, by way of example, in a three-dimensional view, formingcombined output light by combining first out-coupled light with secondout-coupled light,

FIG. 4 c shows, by way of example, in cross-sectional side view, adisplay apparatus, which comprises the expander device,

FIG. 5 shows, by way of example, in a front view, dimensions of theexpander device,

FIG. 6 a shows, by way of example, mapping of wave vector values forblue light, which propagates along the first route of the expanderdevice,

FIG. 6 b shows, by way of example, mapping of wave vector values for redlight, which propagates along the first route of the expander device,

FIG. 6 c shows, by way of example, mapping of wave vector values forblue light of the corner points of the displayed image,

FIG. 6 d shows, by way of example, mapping of wave vector values forblue light of the corner points of the displayed image,

FIG. 6 e shows, by way of example, mapping of wave vector values for redlight of the corner points of the displayed image,

FIG. 6 f shows, by way of example, mapping of wave vector values for redlight of the corner points of the displayed image,

FIG. 6 g shows, by way of example, in a cross-sectional side view,forming first guided light by coupling input light into the substrate,wherein the inclination angle of the first guided light is close to thecritical angle of total internal reflection,

FIG. 6 h shows, by way of example, in a cross-sectional side view,forming first guided light by coupling input light into the substrate,wherein the inclination angle of the first guided light is close to 90degrees,

FIG. 6 i shows, by way of example, the inclination angle of wave vectorof first guided light as a function of input angle of the wave vector ofinput light,

FIG. 7 a shows, by way of example, mapping of wave vector values forblue light, which propagates along the second route of the expanderdevice,

FIG. 7 b shows, by way of example, mapping of wave vector values for redlight, which propagates along the second route of the expander device,

FIG. 7 c shows, by way of example, mapping of wave vector values forblue light of the corner points of the displayed image,

FIG. 7 d shows, by way of example, mapping of wave vector values forblue light of the corner points of the displayed image,

FIG. 8 a shows, by way of example, in a front view, propagation of lightof a first corner point,

FIG. 8 b shows, by way of example, in a front view, propagation of lightof a center point,

FIG. 8 c shows, by way of example, in a front view, propagation of lightof a third corner point,

FIG. 8 d shows, by way of example, in a front view, propagation of lightof a second corner point, and

FIG. 8 e shows, by way of example, in a front view, propagation of lightof a fourth corner point.

DESCRIPTION OF THE EMBODIMENTS

Referring to FIGS. 2 a to 2 e , an optical engine ENG1 may comprise adisplay DISP1 and collimating optics LNS1. The display DISP1 may bearranged to display an input image IMG0. The display DISP1 may also becalled e.g. as a micro display. The display DISP1 may also be callede.g. as a spatial intensity modulator. The input image IMG0 may also becalled e.g. as primary image.

The input image IMG0 may comprise a center point P0 and four cornerpoints P1, P2, P3, P4. P1 may denote an upper left corner point. P2 maydenote an upper right corner point. P3 may denote a lower left cornerpoint. P4 may denote a lower right corner point. The input image IMG0may comprise e.g. the graphical characters “F”, “G”, and “H”.

The input image IMG0 may be a multi-color image. The input image IMG0may be e.g. an RGB image, which may comprise a red partial image, agreen partial image, and a blue partial image. Each image point mayprovide e.g. red light, green light and/or blue light. The light of ared light beam may have red color, e.g. at a wavelength 650 nm. Thelight of a green light beam may have a green color, e.g. at a wavelength510 nm. The light of a blue light beam may have a blue color, e.g. at awavelength 470 nm. In particular, light of a corner point of themulti-color image IMG0 may comprise red light and blue light.

The optical engine ENG1 may provide input light IN1, which may comprisea plurality of substantially collimated light beams (B0). Each red lightbeam may propagate in a different direction and may correspond to adifferent point of the input image IMG0. For example, a red light beamB0 _(P1,B) may correspond to an image point P1, and may propagate in thedirection of a wave vector k0 _(P1,R).

Also a blue light beam (B0 _(P1,B)) may correspond to the same imagepoint P1, and may propagate in the direction of a wave vector (k0_(P1,B)).

The input light IN1 may be formed such that the direction (k0 _(P1,B))of propagation of the blue light beam (B0 _(P1,B)) corresponding to afirst corner point P1 of the input image IMG0 may be parallel with thedirection k0 _(P1,R) of propagation of the red light beam B0 _(P1,R).

The input light IN1 may be formed such that the direction (k0 _(P2,B))of propagation of a blue light beam (B0 _(P1,B)) corresponding to asecond corner point P2 of the input image IMG0 may be parallel with thedirection (k0 _(P2,R)) of propagation of a red light beam (B0 _(P2,R)),which corresponds to said second corner point P2.

A red light beam B0 _(P2,R) may correspond to an image point P2, and maypropagate in the direction of a wave vector k0 _(P2,R). A red light beamB0 _(P3,R) may correspond to an image point P3, and may propagate in thedirection of a wave vector k0 _(P3,R). A red light beam B0 _(P4,R) maycorrespond to an image point P4, and may propagate in the direction of awave vector k0 _(P4,R). A red light beam B0 _(P0,R) may correspond to acentral image point P1, and may propagate in the direction of a wavevector k0 _(P0,R).

The wave vector (k) of light may be defined as the vector having adirection of propagation of said light, and a magnitude given by 2π/λ,where λ is the wavelength of said light.

Referring to FIG. 2 f , output light OUT1 may comprise a plurality ofoutput light beams, which may correspond to a displayed virtual imageVIMG1. Each output beam may correspond to a point of the image. Forexample a red light beam propagating in a direction of a wave vector k3_(P0,R) may correspond to a point P0′ of the image VIMG1. A red lightbeam propagating in a direction of a wave vector k3 _(P1,R) maycorrespond to a point P1′ of the image VIMG1. A red light beampropagating in a direction of a wave vector k3 _(P2,R) may correspond toa point P2′ of the image VIMG1. A red light beam propagating in adirection of a wave vector k3 _(P3,R) may correspond to a point P3′. Ared light beam propagating in a direction of a wave vector k3 _(P4,R)may correspond to a point P4′.

The expander device EPE1 may form the output light OUT1 by expanding theexit pupil of the optical engine ENG1. The output light OUT1 maycomprise a plurality of output light beams, which correspond to thedisplayed virtual image VIMG1. The output light OUT1 may impinge on theeye EYE1 of an observer such that the observer may see the displayedvirtual image VIMG1.

The displayed virtual image VIMG1 may have a center point P0′ and fourcorner points P1′, P2′, P3′, P4′. The input light IN1 may comprise aplurality of partial light beams corresponding to the points P0, P1, P2,P3, P4 of the input image IMG0. The expander device EPE1 may form thepoint P0′ of the displayed virtual image VIMG1 by diffracting andguiding light of the point P0 of the input image IMG0. The expanderdevice EPE1 may form the points P1′, P2′, P3′, P4′ by diffracting andguiding light of the points P1, P2, P3, P4, respectively.

The expander device EPE1 may form output light OUT1, which comprises aplurality of light beams propagating in different directions specifiedby the wave vectors k3 _(P0,R), k3 _(P1,R), k3 _(P2,R), k3 _(P3,R), k3_(P4,R).

A red light beam corresponding to the point P0′ of the displayed virtualimage VIMG1 has a wave vector k3 _(P0,R). A red light beam correspondingto the point P1′ has a wave vector k3 _(P1,R). A red light beamcorresponding to the point P2′ has a wave vector k3 _(P2,R). A red lightbeam corresponding to the point P3′ has a wave vector k3 _(P3,R). A redlight beam corresponding to the point P4′ has a wave vector k3 _(P4,R).

The expander device EPE1 may be arranged to operate such that the wavevector k3 _(P1,R) is parallel with the wave vector k0 _(P1,R) of redlight of the point P1 in the input light IN1. The wave vector k3 _(P0,R)may be parallel with the wave vector k0 _(P0,R) of the point P0. Thewave vector k3 _(P2,R) may be parallel with the wave vector k0 _(P2,R)of the point P2. The wave vector k3 _(P3,R) may be parallel with thewave vector k0 _(P3,R) of the point P3. The wave vector k3 _(P4,R) maybe parallel with the wave vector k0 _(P4,R) of the point P4.

Referring to FIGS. 2 g and 2 h , the displayed virtual image VIMG1 hasan angular width Δφ and an angular height Δθ.

The displayed virtual image VIMG1 may have a first corner point P1′ e.g.at the left-hand side of the image VIMG1, and a second corner point P2′e.g. at the right-hand side of the image VIMG1. The angular width Δφ ofthe virtual image VIMG1 may be equal to the horizontal angle between thewave vectors k3 _(P1,R), k3 _(P2,R) of the corner points P1′, P2′.

The displayed virtual image VIMG1 may have an upper corner point P1′ anda lower corner point P3′. The angular height Δθ of the virtual imageVIMG1 may be equal to the vertical angle between the wave vectors k3_(P1,R), k3 _(P3,R) of the corner points P1′, P3′.

The two routes of the expander device EPE1 may allow displaying a widemulti-color virtual image VIMG1. The two routes of the expander deviceEPE1 may allow displaying a multi-color virtual image VIMG1, which hasan extended width Δφ.

The direction of a wave vector may be specified e.g. by orientationangles φ and θ. The angle φ may denote an angle between the wave vectorand a reference plane REF1. The reference plane REF1 may be defined e.g.by the directions SZ and SY. The angle θ may denote an angle between thewave vector and a reference plane REF2. The reference plane REF2 may bedefined e.g. by the directions SZ and SX.

Referring to FIG. 3 a , the expander device EPE1 may comprise asubstantially planar waveguide plate SUB1, which in turn may comprise adiffractive in-coupling element DOE1, a first diffractive expanderelement DOE2 a, a second diffractive expander element DOE2 b, and adiffractive out-coupling element DOE3. The gratings may be e.g. on firstand/or second surface of the waveguide plate SUB1.

The in-coupling element DOE1 may receive input light IN1, and theout-coupling element DOE3 may provide output light OUT1. The input lightIN1 may comprise a plurality of light beams propagating in differentdirections. The output light OUT1 may comprise a plurality of expandedlight beams formed from the light beams (B0) of the input light IN1.

The width w_(OUT1) of the light beams of the output light OUT1 may begreater than the width w_(IN1) of the light beams of the input lightIN1. The expander device EPE1 may expand the input light IN1 in twodimensions (e.g. in the horizontal direction SX and in the verticaldirection SY). The expansion process may also be called as exit pupilexpansion. The expander device EPE1 may be called as a beam-expanderdevice or as an exit pupil expander.

The in-coupling element DOE1 may form first guided light B1 a and secondguided light B1 b by diffracting input light IN1. The first guided lightB1 a and the second guided light B1 b may be waveguided within theplanar waveguide plate SUB1. The first guided light B1 a and the secondguided light B1 b may be confined to the plate SUB1 by total internalreflection.

The term “guided” may mean that the light propagates within the planarwaveguide plate SUB1 so that the light is confined to the plate by totalinternal reflection (TIR). The term “guided” may mean the same as theterm “waveguided”.

The in-coupling element DOE1 may couple the input light IN1 to propagateto the out-coupling element DOE3 via two different routes, i.e. via thefirst expander element DOE2 a and via the second expander element DOE2b. The in-coupling element DOE1 may be optically coupled to theout-coupling element DOE3 via the first expander element DOE2 a. Thein-coupling element DOE1 may be optically coupled to the out-couplingelement DOE3 also via the second expander element DOE2 b. The expanderdevice EPE1 may provide a first route from the element DOE1 via theelement DOE2 a to the element DOE3. The expander device EPE1 may providea second route from the element DOE1 via the element DOE2 b to theelement DOE3. The first route may mean an optical path from thein-coupling element DOE1 to the out-coupling element DOE3 via the firstexpander element DOE2 a. The second route may mean an optical path fromthe in-coupling element DOE1 to the out-coupling element DOE3 via thesecond expander element DOE2 b.

The first guided light B1 a may propagate from the in-coupling elementDOE1 to the first expander element DOE2 a mainly in a first directionDIR1 a. The first expander element DOE2 a may form third guided light B2a by diffracting the first guided light B1 a. The transverse dimensionof the third guided light B2 a may be greater than the correspondingtransverse dimension of the input light IN1. The third guided light B2 amay also be called e.g. as expanded guided light B2 a.

The expanded guided light B2 a may propagate from the first expanderelement DOE2 a to the out-coupling element DOE3. The expanded guidedlight B2 a may be confined to the plate SUB1 by total internalreflection.

The out-coupling element DOE3 may form first output light OB3 a bydiffracting the expanded guided light B2 a.

The second guided light B1 b may propagate from the in-coupling elementDOE1 to the second expander element DOE2 b mainly in a second directionDIR1 b. The second expander element DOE2 b may form fourth guided lightB2 b by diffracting the second guided light B1 b. A transverse dimensionof the fourth guided light B2 a may be greater than the correspondingtransverse dimension of the input light IN1. The fourth guided light B2b may also be called e.g. as expanded guided light B2 b.

The expanded guided light B2 b may propagate from the second expanderelement DOE2 b to the out-coupling element DOE3. The expanded guidedlight B2 b may be confined to the plate SUB1 by total internalreflection. The out-coupling element DOE3 may form second output lightOB3 b by diffracting the expanded guided light B2 b.

The out-coupling element DOE3 may diffract guided light B2 a receivedfrom the first expander element DOE2 a, and the out-coupling elementDOE3 may diffract guided light B2 b received from the second expanderelement DOE2 b.

The direction DIR1 a may mean the average propagation direction of thefirst guided light B1 a. The direction DIR1 a may denote the centralaxis of propagation of the first guided light B1 a.

The direction DIR1 b may mean the average propagation direction of thesecond guided light B1 b. The direction DIR1 b may denote the centralaxis of propagation of the second guided light B1 b.

The angle γ_(1ab) between the first direction DIR1 a and the seconddirection DIR1 b may be e.g. in the range of 60° to 120°.

The expanded guided light B2 a may propagate in a third direction DIR2a, which may be e.g. approximately parallel with the second directionDIR1 b. The expanded guided light B2 b may propagate in a fourthdirection DIR2 b, which may be e.g. approximately parallel with thefirst direction DIR1 a.

The plate SUB1 may comprise one or more optically isolating elementsISO1 to prevent direct optical coupling between the first expanderelement DOE2 a and the second expander element DOE2 b. An isolatingelement ISO1 may be formed e.g. by depositing (black) absorbing materialon the surface of the plate, by adding (black) absorbing material into aregion of the plate, and/or by forming one or more openings into theplate.

SX, SY and SZ denote orthogonal directions. The plate SUB1 may beparallel with a plane defined by the directions SX and SY.

Referring to FIG. 3 b , the first expander element DOE2 a may bearranged to distribute the guided light B2 a to a first out-couplingregion REG3 a of the out-coupling element DOE3. The first out-couplingregion REG3 a may diffract the guided light B2 a out of the plate SUB1.The second expander element DOE2 b may be arranged to distribute theguided light B2 b to a second out-coupling region REG3 b of theout-coupling element DOE3. The second out-coupling region REG3 b maydiffract the guided light B2 b out of the plate SUB1.

The first out-coupling region REG3 a may overlap with the secondout-coupling region REG3 b. The common region COM1 of the firstout-coupling region REG3 a and the second out-coupling region REG3 b maydiffract guided light B2 a and guided light B2 b out of the plate SUB1.The area of the common region COM1 may be e.g. greater than 50% of theone-sided area of the out-coupling element DOE3, advantageously greaterthan 70%.

Referring to FIGS. 4 a to 4 c , the expander device EPE1 may form outputlight OUT1 by diffracting and guiding input light IN1 obtained from anoptical engine ENG1. A display apparatus 500 may comprise the opticalengine ENG1 and the expander device EPE1.

The input light IN1 may comprise a plurality of light beams propagatingin different directions. Each light beam of the input light IN1 maycorrespond to a different point of the input image IMG0. The outputlight OUT1 may comprise a plurality of light beams propagating indifferent directions. Each light beam of the output light OUT1 maycorrespond to a different point of the displayed virtual image VIMG1.The expander device EPE1 may form the output light OUT1 from the inputlight IN1 such that the directions and the intensities of the lightbeams of the output light OUT1 correspond to the points of the inputimage IMG0.

A light beam of the input light IN1 may correspond to a single imagepoint (P0) of a displayed image. The expander device EPE1 may form anoutput light beam from a light beam of the input light IN1 such that thedirection (k_(3,P0,R)) of the output light beam is parallel with thedirection (k_(0,P0,R)) of the corresponding light beam of the inputlight IN1.

The display apparatus 500 may comprise an optical engine ENG1 to form aprimary image IMG0 and to convert the primary image IMG0 into aplurality of light beams of the input light IN1. The engine ENG1 may beoptically coupled to the in-coupling element DOE1 of the expander EPE1.The input light IN1 may be optically coupled to the in-coupling elementDOE1 of the expander device EPE1. The apparatus 500 may be e.g. displaydevice for displaying virtual images. The apparatus 500 may be a neareye optical device.

The expander device EPE1 may carry virtual image content from the lightengine ENG1 to the front of a user's eye EYE1. The expander device EPE1may expand the viewing pupil, thus enlarging the eye box.

The engine ENG1 may comprise a micro-display DISP1 to generate a primaryimage IMG0. The micro-display DISP1 may comprise a two-dimensional arrayof light-emitting pixels. The display DISP1 may generate a primary imageIMG0 e.g. at a resolution of 1920×1080 (Full HD). The display DISP1 maygenerate a primary image IMG0 e.g. at a resolution of 1920×1080 (FullHD). The display DISP1 may generate a primary image IMG0 e.g. at aresolution of 3840×2160 (4K UHD). The primary image IMG0 may comprise aplurality of image points P0, P1, P2, . . . . The engine ENG1 maycomprise collimating optics LNS1 to form a different light beam fromeach image pixel. The engine ENG1 may comprise collimating optics LNS1to form a substantially collimated light beam from light of an imagepoint P0. The light beam corresponding to the image point P0 maypropagate in the direction specified by a wave vector k0 _(P0,R). Alight beam corresponding to a different image point P1 may propagate ina direction k0 _(P1,R) which is different from the direction k0 _(P0,R).

The engine ENG1 may provide a plurality of light beams corresponding tothe generated primary image IMG0. The one or more light beams providedby the engine ENG1 may be coupled to the expander EPE1 as input lightIN1.

The engine ENG1 may comprise e.g. one or more light emitting diodes(LED). The display DISP1 may comprise e.g. one or more micro displayimagers, such as liquid crystal on silicon (LCOS), liquid crystaldisplay (LCD), digital micromirror device (DMD).

The out-coupling element DOE3 may form first output light OB3 a bydiffracting guided light B2 a received from the first expander elementDOE2 a. The out-coupling element DOE3 may form second output light OB3 bby diffracting guided light B2 b received from the second expanderelement DOE2 b. The out-coupling element DOE3 may form combined outputlight OUT1 by combining the first output light OB3 a with the secondoutput light OB3 b.

The expander device EPE1 may be arranged to operate such that thedirection of light of a given image point (e.g. P0) in the first outputlight OB3 a is parallel with the direction of light of said given imagepoint (P0) in the second output light OB3 b. Consequently, the combiningthe first output light OB3 a with the second output light OB3 b may forma combined light beam, which corresponds to said given image point (P0).

Each element DOE1, DOE2 a, DOE2 b, DOE3 may comprise one or morediffraction gratings to diffract light as described.

The grating periods (d) and the orientations (β) of the diffractiongratings of the optical elements DOE1, DOE2 a, DOE2 b, DOE3 may beselected such that the direction of each light beam of the output lightOUT1 may be parallel with the direction of the corresponding light beamof the input light IN1.

The grating periods (d) and the direction (β) of the grating vectors mayfulfill e.g. the condition that the vector sum(m_(1a)V_(1a)+m_(2a)V_(2a)+m_(3a)V_(3a)) is zero for predeterminedintegers m_(1a), m_(2a), m_(3a). V_(1a) denotes a grating vector of theelement DOE1. V_(2a) denotes a grating vector of the element DOE2 a.V_(3a) denotes a grating vector of the element DOE3. The value of theseintegers is typically +1 or −1. The value of the integer m_(1a) may bee.g. +1 or −1. The value of the integer m_(2a) may be e.g. +1 or −1. Thevalue of the integer m_(3a) may be e.g. +1 or −1.

The grating periods (d) and the direction (β) of the grating vectors mayfulfill e.g. the condition that the vector sum(m_(1b)V_(1b)+m_(2b)V_(2b)+m_(3b)V_(3b)) is zero for predeterminedintegers m_(1b), m_(2b), m_(3b). V_(1b) denotes a grating vector of theelement DOE1. V_(2b) denotes a grating vector of the element DOE2 b.V_(3b) denotes a grating vector of the element DOE3. The value of theseintegers is typically +1 or −1. The value of the integer m_(1b) may bee.g. +1 or −1. The value of the integer m_(2b) may be e.g. +1 or −1. Thevalue of the integer m_(3b) may be e.g. +1 or −1.

The waveguiding plate may have a thickness t_(SUB1). The waveguidingplate comprises a planar waveguiding core. In an embodiment, the plateSUB1 may optionally comprise e.g. one or more cladding layers, one ormore protective layers, and/or one or more mechanically supportinglayers. The thickness t_(SUB1) may refer to the thickness of a planarwaveguiding core of the plate SUB1.

The expander device EPE1 may expand a light beam in two transversedirections, in the direction SX and in the direction SY. The width (indirection SX) of the output light beam OUT1 may be greater than thewidth of the input light beam IN1, and the height (in direction SY) ofthe output light beam OUT1 may be greater than the height of the inputlight beam IN1.

The expander device EPE1 may be arranged to expand a viewing pupil ofthe virtual display device 500, so as to facilitate positioning of aneye EYE1 with respect to the virtual display device 500. A humanobserver may see a displayed virtual image VIMG1 in a situation wherethe output light OUT1 is arranged to impinge on an eye EYE1 of the humanviewer. The output light OUT1 may comprise one or more output lightbeams, wherein each output light beam may correspond to a differentimage point (P0′, P1′) of a displayed virtual image VIMG1. The engineENG1 may comprise a micro display DISP1 for displaying a primary imageIMG0. The engine ENG1 and the expander device EPE1 may be arranged todisplay the virtual image VIMG1 by converting the primary image IMG0into a plurality of input light beams (e.g. B0 _(P0,R), B0 _(P1,R), B0_(P2,R), B0 _(P3,R), B0 _(P4,R), . . . B0 _(P0,B), B0 _(P1,B), B0_(P2,B), B0 _(P3,B), B0 _(P4,B), . . . ), and by forming output lightbeams OUT1 from the input beams by expanding the input beams. Forexample, the notation B0 _(P2,R) may mean an input light beam, whichcorresponds to an image point P2 and which has red (R) color. Forexample, the notation B0 _(P2,B) may mean an input light beam, whichcorresponds to the image point P2 and which has blue (B) color. Theinput light beams may together constitute input light IN1. The inputlight IN1 may comprise a plurality of input light beams (e.g. B0_(P0,R), B0 _(P1,R), B0 _(P2,R), B0 _(P3,R), B0 _(P4,R), . . . B0_(P0,B), B0 _(P1,B), B0 _(P2,B), B0 _(P3,B), B0 _(P4,B), . . . ).

The output light OUT1 may comprise a plurality of output light beamssuch that each output light beam may form a different image point (P0′,P1′) of the virtual image VIMG1. The primary image IMG0 may be represente.g. graphics and/or text. The primary image IMG0 may be represent e.g.video. The engine ENG1 and the expander device EPE1 may be arranged todisplay the virtual image VIMG1 such that each image point (P0′, P1′) ofthe virtual image VIMG1 corresponds to a different image point of theprimary image IMG0.

The plate SUB1 may have a first major surface SRF1 and a second majorsurface SRF2. The surfaces SRF1, SRF2 may be substantially parallel withthe plane defined by the directions SX and SY.

Referring to FIG. 5 , each element DOE1, DOE2 a, DOE2 b, DOE3 maycomprise one or more diffraction gratings to diffract light asdescribed. For example, the element DOE1 may comprise one or moregratings G1 a, G1 b. For example, the element DOE2 a may comprise agrating G2 a. For example, the element DOE2 b, may comprise a grating G2b. For example, the element DOE3 may comprise one or more gratings G3 a,G3 b.

A grating period (d) of a diffraction grating and the orientation (β) ofthe diffractive features of the diffraction grating may be specified bya grating vector V of said diffraction grating. The diffraction gratingcomprises a plurality of diffractive features (F) which may operate asdiffractive lines. The diffractive features may be e.g. microscopicridges or grooves. The diffractive features may also be e.g. microscopicprotrusions (or recesses), wherein adjacent rows of protrusions (orrecesses) may operate as diffractive lines. The grating vector V may bedefined as a vector having a direction perpendicular to diffractivelines of the diffraction grating and a magnitude given by 2π/d, where dis the grating period.

The in-coupling element DOE1 may have grating vectors V_(1a), V_(1b).The first expander element DOE2 a may have a grating vector V_(2a). Thesecond expander element DOE2 b may have a grating vector V_(2b). Theout-coupling element DOE3 may have grating vectors V_(3a), V_(3b).

The grating vector V_(1a) has a direction β_(1a) and a magnitude2π/d_(1a). The grating vector V_(1b) has a direction β_(1b) and amagnitude 2πn/d_(1b). The grating vector V_(2a) has a direction β₂ and amagnitude 2π/d_(2a). The grating vector V_(2b) has a direction β_(2b)and a magnitude 2π/d_(2b). The grating vector V_(3a) has a directionβ_(3a) and a magnitude 2π/d_(3a). The grating vector V_(3b) has adirection β_(3b) and a magnitude 2π/d_(3b). The direction (β) of agrating vector may be specified e.g. by the angle between said vectorand a reference direction (e.g. direction SX).

The grating periods (d) and the orientations (β) of the diffractiongratings of the optical elements DOE1, DOE2 a, DOE3 may be selected suchthat the direction (k3 _(P0,R)) of propagation of light of the centerpoint P0 in the first output light OB3 a is parallel with the direction(k0 _(P0,R)) of propagation of light of the center point P0 in the inputlight IN1.

The grating periods (d) and the orientations (β) of the diffractiongratings of the optical elements DOE1, DOE2 b, DOE3 may be selected suchthat the direction (k3 _(P0,R)) of propagation of light of the centerpoint P0 in the second output light OB3 b is parallel with the direction(k0 _(P0,R)) of propagation of light of the center point P0 in the inputlight IN1.

The grating periods (d) and the orientations (β) of the diffractiongratings of the optical elements DOE1, DOE2 a, DOE2 b, DOE3 may beselected such that the direction (k3 _(P0,R)) of propagation of light ofthe center point P0 in the combined output light OUT1 is parallel withthe direction (k0 _(P0,R)) of propagation of light of the center pointP0 in the input light IN1.

An angle between the directions of the grating vectors V_(1a), V_(1b) ofthe in-coupling element DOE1 may be e.g. in the range of 60° to 120°.

The first grating period d_(1a) of the element DOE1 may be differentfrom the second grating period dib of the element DOE1, for optimizingthe first route for a first color, and for optimizing the second routefor a second different color.

The first grating period d_(3a) of the element DOE3 may be differentfrom the second grating period d_(3b) of the element DOE3, foroptimizing the first route for a first color, and for optimizing thesecond route for a second different color.

The first grating period d_(1a) of the element DOE1 may be differentfrom the second grating period dib of the element DOE1, e.g. foroptimizing the first route for blue color, and for optimizing the secondroute for red color.

The first grating period d_(3a) of the element DOE3 may be differentfrom the second grating period d_(3b) of the element DOE3, e.g. foroptimizing the first route for blue color, and for optimizing the secondroute for red color.

The grating periods (d) and the direction (β) of the grating vectors mayfulfill e.g. the condition that the vector sum(m_(1a)V_(1a)+m_(2a)V_(2a)+m_(3a)V_(3a)) is zero for predeterminedintegers m_(1a), m_(2a), m_(3a). V_(1a) denotes a grating vector of theelement DOE1. V_(2a) denotes a grating vector of the element DOE2 a.V_(1a) denotes a grating vector of the element DOE3. The value of theseintegers is typically +1 or −1. The value of the integer m_(1a) may bee.g. +1 or −1. The value of the integer m_(2a) may be e.g. +1 or −1. Thevalue of the integer m_(3a) may be e.g. +1 or −1.

The grating periods (d) and the direction (β) of the grating vectors mayfulfill e.g. the condition that the vector sum(m_(1b)V_(1b)+m_(2b)V_(2b)+m_(3b)V_(3b)) is zero for predeterminedintegers m_(1b), m_(2b), m_(3b). V_(1b) denotes a grating vector of theelement DOE1. V_(2b) denotes a grating vector of the element DOE2 b.V_(3b) denotes a grating vector of the element DOE3. The value of theseintegers is typically +1 or −1. The value of the integer m_(1b) may bee.g. +1 or −1. The value of the integer m_(2b) may be e.g. +1 or −1. Thevalue of the integer m_(3b) may be e.g. +1 or −1.

The first element DOE1 may have a first grating vector V1 a to form thefirst guided light B1 a to the direction DIR1 a and a second gratingvector V1 b to form the second guided light B1 b to the direction DIR1b. The first element DOE1 may have first diffractive features F1 a toprovide a first grating which has a grating period d_(1a) and anorientation β_(1a) with respect to a reference direction SX. The firstelement DOE1 may have second diffractive features F1 b to provide asecond grating which has a grating period d_(1b) and an orientationβ_(1b) with respect to the reference direction SX. The first elementDOE1 may be implemented e.g. by a crossed grating or by two lineargratings. The first element DOE1 may e.g. comprise a first region, whichcomprises first features F1 a, and the first element DOE1 may comprise asecond region, which comprises F1 b.

A first linear grating having features F1 a may be implemented on afirst side (e.g. on an input side SRF1) of the plate SUB1, and a secondlinear grating having features F1 b may be implemented on the secondside (e.g. on an output side SRF2) of the plate SUB1. The diffractivefeatures may be e.g. microscopic ridges or microscopic protrusions.

The expander element DOE2 a may have a grating vector V2 a to form thethird guided light B2 a by diffracting the first guided light B1 a. Theexpander element DOE2 a may have diffractive features F2 a to provide agrating G2 a which has a grating period d_(2a) and an orientation β_(2a)with respect to the reference direction SX.

The expander element DOE2 b may have a grating vector V2 b to form thefourth guided light B2 b by diffracting the second guided light B1 b.The expander element DOE2 b may have diffractive features F2 b toprovide a grating G2 b which has a grating period deb and an orientationβ_(2b) with respect to the reference direction SX.

The out-coupling element DOE3 may have a first grating vector V3 a tocouple the expanded light B2 a out of the plate SUB1. The out-couplingelement DOE3 may have a second grating vector V3 b to couple theexpanded light B2 b out of the plate SUB1. The out-coupling element DOE3may have diffractive features F3 a to provide a grating G3 a which has agrating period d_(3a) and an orientation β_(3a) with respect to thereference direction SX. The out-coupling element DOE3 may havediffractive features F3 b to provide a grating G3 b which has a gratingperiod dab and an orientation β_(3b) with respect to the referencedirection SX. The out-coupling element DOE3 may be implemented e.g. by acrossed grating or by two linear gratings. A first linear grating G3 ahaving features F3 a may be implemented on a first side (e.g. SRF1) ofthe plate SUB1, and a second linear grating G3 b having features F3 bmay be implemented on the second side (e.g. SRF2) of the plate SUB1.

The in-coupling element DOE1 may have a width w₁ and a height h₁. Thefirst expander element DOE2 a may have a width w_(2a) and a heighth_(2a). The second expander element DOE2 b may have a width web and aheight h_(2b). The out-coupling element DOE3 may have a width w₃ and aheight h₃.

The width may denote a dimension in the direction SX, and the height maydenote a dimension in the direction SY. The out-coupling element DOE3may be e.g. substantially rectangular. The sides of the out-couplingelement DOE3 may be aligned e.g. with the directions SX and SY.

The width w_(2a) of the expander element DOE2 a may be substantiallygreater than the width w₁ of the in-coupling element DOE1. The width ofan expanded guided light beam B2 a may be substantially greater than thewidth w1 of the in-coupling element DOE1.

The plate SUB1 may comprise or consist essentially of transparent solidmaterial. The plate SUB1 may comprise e.g. glass, polycarbonate orpolymethyl methacrylate (PMMA). The diffractive optical elements DOE1,DOE2 a, DOE2 b, DOE3 may be formed e.g. by molding, embossing, and/oretching. The elements DOE1, DOE2 a, DOE2 b, DOE3 may be implemented e.g.by one or more surface diffraction gratings or by one or more volumediffraction gratings.

The spatial distribution of diffraction efficiency may be optionallytailored e.g. by selecting the local elevation of the microscopicdiffractive features F. The elevation of the microscopic diffractivefeatures F of the out-coupling element DOE3 may be optionally selectedso as to further homogenize the intensity distribution of the outputlight OUT1.

FIG. 6 a shows, by way of example, mapping of wave vectors for bluelight, which propagates within the waveguiding plate SUB1 along thefirst route. The first route may be e.g. a clockwise route. The wavevectors of the input light IN1 may be within a region BOX0 of the wavevector space defined by elementary wave vectors k_(x) and k_(y). Eachcorner of the region BOX0 may represent a wave vector of light of acorner point of input image IMG0 (FIG. 7 a ).

The wave vectors of the first guided light B1 a may be within a regionBOX1 a.

The wave vectors of the third guided light B2 a may be within a regionBOX2 a.

The wave vectors of the first output light OB3 a may be within a regionBOX3.

The in-coupling element DOE1 may form the first guided light B1 a bydiffracting the input light IN1. The diffraction may be represented byadding the grating vector m_(1a) V1 a of the in-coupling element DOE1 tothe wave vectors of the input light IN1. The wave vectors of the firstguided light B1 a may be determined by adding the grating vector m_(1a)V1 a to the wave vectors of the input light IN1. The wave vectors of thethird guided light B2 a may be determined by adding the grating vectorm_(2a) V2 a to the wave vectors of the first guided light B1 a. The wavevectors of the out-coupled light OB3 a may be determined by adding thegrating vector m_(3a) V3 a to the wave vectors of the second guidedlight B2 a.

BND1 denotes a first boundary for fulfilling the criterion of totalinternal reflection (TIR) in the waveguiding plate SUB1. BND2 denotes asecond boundary of maximum wave vector in the waveguiding plate SUB1.The maximum wave vector may be determined by the refractive index of thesubstrate. Light may be waveguided in the plate SUB1 only when the wavevector of said light is in the region ZONE1 between the first boundaryBND1 and the second boundary BND2. The light may leak out of the plateor not propagate at all if the wave vector of the light is outside theregion ZONE1.

The grating period d_(1a) of the in-coupling element DOE1 may beselected e.g. such that all wave vectors of the first blue guided lightB1 a are within the region ZONE1 defined by the boundaries BND1, BND2.

FIG. 6 b shows, by way of example, mapping of wave vectors for redlight, which propagates within the waveguiding plate SUB1 along thefirst route.

Now, if the grating period d_(1a) of the in-coupling element DOE1 hasbeen selected such that all wave vectors of the first blue guided lightB1 a are within the region ZONE1, then the wave vectors of red light ofsome corner points may be outside the region ZONE1. In other words, thewaveguiding plate SUB1 cannot confine or guide the red light of somecorner points of the input image IMG0.

Wave vectors residing within the sub-region FAIL1 of the region BOX1 amay correspond to a situation where the input element DOE1 fails to formguided light by diffracting input light. In other words, the diffractionequation does not provide a real practical solution for wave vectorsresiding within the sub-region FAIL1 of the region BOX1 a. Thus, forsome image points, it is not possible to couple red light into thesubstrate, in a situation where the wave vectors of guided light wouldbe outside the region ZONE1.

For some (other) image points, the leaking of the red light may limitthe angular width of the displayed virtual image VIMG1, in a situationwhere the wave vectors of guided light would be outside the regionZONE1.

Thus, the boundaries BND1, BND2 of the region ZONE1 may limit theangular width (Δφ) of the displayed virtual image VIMG1. Formation of awave vector, which is outside the region ZONE1 may mean leakage of lightout of the substrate or failed coupling of light into the substrate.

k_(x) denotes a direction in the wave vector space, wherein thedirection k_(x) is parallel with the direction SX of the real space.k_(y) denotes a direction in the wave vector space, wherein thedirection k_(y) is parallel with the direction SY of the real space. Thesymbol k_(z) (not shown in the drawings) denotes a direction in the wavevector space, wherein the direction k_(z) is parallel with the directionSZ of the real space. A wave vector k may have components in thedirections k_(x), k_(y), and/or k_(z).

FIGS. 6 c and 6 d show, by way of example, the wave vectors of bluelight of the image points (P0, P1, P2, P3, P4) in the wave vector space.

FIGS. 6 e and 6 f show, by way of example, the wave vectors of red lightof the image points (P0, P1, P2, P3, P4) in the wave vector space.

FIG. 6 g shows, by way of example, in a cross-sectional side view,forming first guided light by coupling input light into the substrate,wherein the inclination angle φ_(k1) of the first guided light is closeto the critical angle φ_(CR,SUB1) of total internal reflection. Thesituation of FIG. 6 g may correspond to operation near the firstboundary BND1 of the region ZONE1.

FIG. 6 h shows, by way of example, in a cross-sectional side view,forming first guided light by coupling input light into the substrate,wherein the inclination angle φ_(k1) of the first guided light is closeto 90 degrees. The situation of FIG. 6 h may correspond to operationnear the second boundary BND2 of the region ZONE1.

The curve CRV1 of FIG. 6 i shows, by way of example, the inclinationangle φ_(k1) of the wave vector k1 of first guided light B1 a as afunction of input angle φ_(k0) of the wave vector k0 of input light B0.The inclination angle φ_(k1) may mean the angle between the wave vectorand the reference plane REF1 defined by the directions SZ and SY. Theinclination angle φ_(k1) may be calculated from the input angle φ_(k0),from the grating period of the input element DOE1, and from therefractive index of the substrate SUB1 e.g. by using the diffractionequation. A first angular limit φ_(BND1) may correspond to a situationwhere the inclination angle φ_(k1) of the first guided light is equal tothe critical angle φ_(CR,SUB1) of total internal reflection. A secondangular limit φ_(BND2) may correspond to a situation where theinclination angle φ_(k1) of the first guided light is equal to 90degrees.

FIG. 7 a shows, by way of example, mapping of wave vectors for bluelight, which propagates within the waveguiding plate SUB1 along thesecond route. The second route may be e.g. a counter-clockwise route.

FIG. 7 b shows, by way of example, mapping of wave vectors for redlight, which propagates within the waveguiding plate SUB1 along thesecond route.

FIGS. 7 c and 7 d show, by way of example, the wave vectors of bluelight of the image points (P0, P1, P2, P3, P4) in the wave vector space.

The grating period dib of the in-coupling element DOE1 may be selectede.g. such that all wave vectors of the second red guided light B1 b arewithin the region ZONE1 defined by the boundaries BND1, BND2.

Now, if the grating period dib of the in-coupling element DOE1 has beenselected such that all wave vectors of the second red guided light B1 bare within the region ZONE1, then the wave vectors of blue light of somecorner points may be outside the region ZONE1. In other words, thewaveguiding plate SUB1 cannot confine the blue light of some cornerpoints of the input image IMG0. The leaking of the blue light may limitthe angular width of the displayed virtual image VIMG1. The wave vectorsresiding in the sub-region LEAK1 of the region BOX2 b may representlight, which is not confined to the substrate by total internalreflection.

However, the expander device EPE1 may be arranged to provide both thefirst route and the second route. The first route may provide the fullwidth (Δφ) of the displayed image VIMG1 at the blue color, and thesecond route may provide the same full width (Δφ) of the displayed imageVIMG1 at the red color. Consequently, the expander device EPE1 may bearranged to display a multi-color virtual image VIMG1, which has thefull width (Δφ).

Consequently, the expander device EPE1 may be arranged to display allcorner points (P1, P2, P3, P4) of the multi-color virtual image VIMG1 inred color and in blue color, wherein said multi-color virtual imageVIMG1 has the full width (Δφ).

Consequently, the angular width (Δφ) of the multi-color virtual imageVIMG1 displayed by using the two routes may be substantially greaterthan a maximum angular width (LIM1) of another multi-color virtualimage, which can be displayed by a comparative device (EPE0) withoutusing the second route.

The expander device EPE1 with the two routes may be arranged to displaya multi-color virtual image VIMG1, which has an extended angular width(Δφ). The first route may be arranged to confine the blue colorcomponents of the input image, while allowing leakage of red light ofone or more corner points of the input image. The second route may bearranged to confine the red color components of the input image, whileallowing leakage of blue light of one or more corner points of the inputimage.

For example, in an instance in which the input light (IN1) correspondsto an input image (IMG0), and the width (Δφ) of the input image (IMG0)is greater than a predetermined limit (LIM1), the in-coupling element(DOE1) may be arranged to provide:

-   -   red light (B1 aP1,R) which corresponds to a first corner point        (P1) of an input image (IMG0),    -   wherein the grating vectors (m1 aV1 a, m2 aV2 a, m3 aV3 a, m1        bV1 b, m2 bV2 b, m3 bV3 b) of the elements (DOE1, DOE2 a, DOE2        b, DOE3) have been selected such that:    -   the red light of the first corner point (P1) is guided from the        in-coupling element (DOE1) to the out-coupling element (DOE3)        via the second expander element (DOE2 b),    -   the red light of the first corner point (P1) is not guided from        the in-coupling element (DOE1) to the out-coupling element        (DOE3) via the first expander element (DOE2 a).

For example, in an instance in which the input light (IN1) correspondsto an input image (IMG0), and the width (Δφ) of the input image (IMG0)is greater than a predetermined limit (LIM1), the in-coupling element(DOE1) may be arranged to provide:

blue light (B1 bP2,B) which corresponds to a second corner point (P2) ofthe input image (IMG0),

-   -   wherein the grating vectors (m1 aV1 a, m2 aV2 a, m3 aV3 a, m1        bV1 b, m2 bV2 b, m3 bV3 b) of the elements (DOE1, DOE2 a, DOE2        b, DOE3) have been selected such that:    -   the blue light of the second corner point (P2) is guided from        the in-coupling element (DOE1) to the out-coupling element        (DOE3) via the first expander element (DOE2 a), and    -   the blue light of the second corner point (P2) is not guided        from the in-coupling element (DOE1) to the out-coupling element        (DOE3) via the second expander element (DOE2 b).

For example, in an instance in which the input light (IN1) correspondsto an input image (IMG0), and the width (Δφ) of the input image (IMG0)is greater than a predetermined limit (LIM1), the in-coupling element(DOE1) may be arranged to provide:

-   -   red light (B1 aP1,R) which corresponds to a first corner point        (P1) of an input image (IMG0),    -   blue light (B1 bP2,B) which corresponds to a second corner point        (P2) of the input image (IMG0),    -   wherein the grating vectors (m1 aV1 a, m2 aV2 a, m3 aV3 a, m1        bV1 b, m2 bV2 b, m3 bV3 b) of the elements (DOE1, DOE2 a, DOE2        b, DOE3) have been selected such that:    -   the red light of the first corner point (P1) is guided from the        in-coupling element (DOE1) to the out-coupling element (DOE3)        via the second expander element (DOE2 b),    -   the red light of the first corner point (P1) is not guided from        the in-coupling element (DOE1) to the out-coupling element        (DOE3) via the first expander element (DOE2 a),    -   the blue light of the second corner point (P2) is guided from        the in-coupling element (DOE1) to the out-coupling element        (DOE3) via the first expander element (DOE2 a), and    -   the blue light of the second corner point (P2) is not guided        from the in-coupling element (DOE1) to the out-coupling element        (DOE3) via the second expander element (DOE2 b).

For example, in an instance in which the input light (IN1) correspondsto an input image (IMG0), and the width (Δφ) of the input image (IMG0)is greater than a predetermined limit (LIM1), the in-coupling element(DOE1) may be arranged to provide:

-   -   red light (B1 aP1,R) which corresponds to a first corner point        (P1) of an input image (IMG0),    -   blue light (B1 aP1,B) which corresponds to the first corner        point (P1) of the input image (IMG0),    -   red light (B1 bP2,R) which corresponds to a second corner point        (P2) of the input image (IMG0),    -   blue light (B1 bP2,B) which corresponds to the second corner        point (P2) of the input image (IMG0),    -   wherein the grating vectors (m1 aV1 a, m2 aV2 a, m3 aV3 a, m1        bV1 b, m2 bV2 b, m3 bV3 b) of the elements (DOE1, DOE2 a, DOE2        b, DOE3) have been selected such that:    -   the red light of the first corner point (P1) is guided from the        in-coupling element (DOE1) to the out-coupling element (DOE3)        via the second expander element (DOE2 b),    -   the red light of the first corner point (P1) is not guided from        the in-coupling element (DOE1) to the out-coupling element        (DOE3) via the first expander element (DOE2 a),    -   the blue light of the first corner point (P1) is guided from the        in-coupling element (DOE1) to the out-coupling element (DOE3)        via the first expander element (DOE2 a),    -   the blue light of the first corner point (P1) is guided from the        in-coupling element (DOE1) to the out-coupling element (DOE3)        via the second expander element (DOE2 a),    -   the red light of the second corner point (P2) is guided from the        in-coupling element (DOE1) to the out-coupling element (DOE3)        via the first expander element (DOE2 a),    -   the red light of the second corner point (P2) is guided from the        in-coupling element (DOE1) to the out-coupling element (DOE3)        via the second expander element (DOE2 b),    -   the blue light of the second corner point (P2) is guided from        the in-coupling element (DOE1) to the out-coupling element        (DOE3) via the first expander element (DOE2 a), and    -   the blue light of the second corner point (P2) is not guided        from the in-coupling element (DOE1) to the out-coupling element        (DOE3) via the second expander element (DOE2 b).

The device EPE1 may be arranged to operate such that the wave vectors ofblue guided light reside within the region ZONE1 in a situation wherethe blue guided light propagates via a first route of the device EPE1,and the device EPE1 may be arranged to operate such that the wavevectors of red guided light reside within the region ZONE1 in asituation where the red guided light propagates via a second route ofthe device EPE1.

FIG. 8 a shows, by way of example, propagation of light of a cornerpoint P1 in the waveguiding plate SUB1.

FIG. 8 b shows, by way of example, propagation of light of a centerpoint P0 in the waveguiding plate SUB1.

FIG. 8 c shows, by way of example, propagation of light of a cornerpoint P3 in the waveguiding plate SUB1.

FIG. 8 d shows, by way of example, propagation of light of a cornerpoint P2 in the waveguiding plate SUB1.

FIG. 8 e shows, by way of example, propagation of light of a cornerpoint P4 in the waveguiding plate SUB1.

The display apparatus 500 may be e.g. a virtual reality device 500. Thedisplay apparatus 500 may be e.g. an augmented reality device 500. Thedisplay apparatus 500 may be a near eye device. The apparatus 500 may bea wearable device, e.g. a headset. The apparatus 500 may comprise e.g. aheadband by which the apparatus 500 may be worn on a user's head. Duringoperation of apparatus 500, the out-coupling element DOE3 may bepositioned e.g. in front of the user's left eye EYE1 or right EYE1. Theapparatus 500 may project output light OUT1 into the user's eye EYE1. Inan embodiment, the apparatus 500 may comprise two engines ENG1 and/ortwo extender devices EPE1 to display stereo images. In an augmentedreality device 500, the viewer may also see real objects and/orenvironment through the expander device EPE1, in addition to thedisplayed virtual images. The engine ENG1 may be arranged to generatestill images and/or video. The engine ENG1 may generate a real primaryimage IMG0 from a digital image. The engine ENG1 may receive one or moredigital images e.g. from an internet server or from a smartphone. Theapparatus 500 may be a smartphone. The displayed image may be viewed bya human. The displayed image may also be viewed e.g. by an animal, or bya machine (which may comprise e.g. a camera).

The term k-vector may mean the same as the term wave vector.

For the person skilled in the art, it will be clear that modificationsand variations of the devices and methods according to the presentinvention are perceivable. The figures are schematic. The particularembodiments described above with reference to the accompanying drawingsare illustrative only and not meant to limit the scope of the invention,which is defined by the appended claims.

What is claimed is:
 1. An optical device (EPE1) comprising a waveguideplate (SUB1), which in turn comprises: an in-coupling element (DOE1) toform first guided light (B1 a) and second guided light (B1 b) bydiffracting input light (IN1) from a first grating (G1 a) and a secondgrating (G1 b) of the in-coupling element (DOE1) respectively, a firstexpander element (DOE2 a) to form third guided light (B2 a) bydiffracting the first guided light (B1 a) from a grating (G2 a) of thefirst expander element (DOE2 a), a second expander element (DOE2 b) toform fourth guided light (B2 b) by diffracting the second guided light(B1 b) from a grating (G2 b) of the second expander element (DOE2 b),and an out-coupling element (DOE3) to form first output light (OB3 a) bydiffracting the third guided light (B2 a) from a first grating (G3 a) ofthe out-coupling element (DOE3), and to form second output light (OB3 b)by diffracting the fourth guided light (B2 b) from a second grating (G3b) of the out-coupling element (DOE3), wherein the out-coupling element(DOE3) is arranged to form combined output light (OUT1) by combining thefirst output light (OB3 a) with the second output light (OB3 b), whereinthe first grating (G1 a) of the in-coupling element (DOE1) has a firstgrating period (d_(1a)) for forming the first guided light (B1 a), andwherein the second grating (G1 b) of the in-coupling element (DOE1) hasa second different grating period (d_(1b)) for forming the second guidedlight (B1 b), and the waveguide plate (SUB1) further comprises one ormore optically isolating elements (ISO1) to prevent direct opticalcoupling between the first expander element (DOE2 a) and the secondexpander element (DOE2 b), wherein the one or more optically isolatingelements (ISO1) are a black absorbing material, wherein the first andsecond gratings (G1 a, G1 b) of the in-coupling element (DOE1), thegrating (G2 a) of the first expander element (DOE2 a), the grating (G2b) of the second expander element (DOE2 b), the first and secondgratings (G3 a, G3 b) of the out-coupling element (DOE3), and the one ormore optically isolating elements (ISO1) are all provided on a samesurface of the waveguide plate (SUB1).
 2. The device (EPE1) of claim 1,wherein the first expander element (DOE2 a) has a third grating period(d_(2a)) for forming the third guided light (B1 a), the second expanderelement (DOE2 b) has a fourth grating period (d_(2b)) for forming thefourth guided light (B2 b), wherein the third grating period (d_(2a)) isdifferent from the fourth grating period (d_(2b)).
 3. The device (EPE1)of claim 1, wherein, in an instance in which the input light (IN1)corresponds to an input image (IMG0), and the width (Δφ) of the inputimage (IMG0) is greater than a predetermined limit (LIM1), thein-coupling element (DOE1) may be arranged to provide: red light (B1 a_(P1,R)) which corresponds to a first corner point (P1) of an inputimage (IMG0), wherein the grating vectors (m_(1a)V_(1a), m_(2a)V_(2a),m_(3a)V_(3a), m_(1b)V_(1b), m_(2b)V_(2b), m_(3b)V_(3b)) of the elements(DOE1, DOE2 a, DOE2 b, DOE3) have been selected such that: the red lightof the first corner point (P1) is guided from the in-coupling element(DOE1) to the out-coupling element (DOE3) via the second expanderelement (DOE2 b), the red light of the first corner point (P1) is notguided from the in-coupling element (DOE1) to the out-coupling element(DOE3) via the first expander element (DOE2 a).
 4. The device (EPE1)according to claim 1, wherein, in an instance in which the input light(IN1) corresponds to an input image (IMG0), and the width (Δφ) of theinput image (IMG0) is greater than a predetermined limit (LIM1), thein-coupling element (DOE1) is arranged to provide: red light (B1 a_(P1,R)) which corresponds to a first corner point (P1) of an inputimage (IMG0), blue light (B1 b _(P2,B)) which corresponds to a secondcorner point (P2) of the input image (IMG0), wherein the grating vectors(m_(1a)V_(1a), m_(2a)V_(2a), m_(3a)V_(3a), m_(1b)V_(1b), m_(2b)V_(2b),m_(3b)V_(3b)) of the elements (DOE1, DOE2 a, DOE2 b, DOE3) have beenselected such that: the red light of the first corner point (P1) isguided from the in-coupling element (DOE1) to the out-coupling element(DOE3) via the second expander element (DOE2 b), the red light of thefirst corner point (P1) is not guided from the in-coupling element(DOE1) to the out-coupling element (DOE3) via the first expander element(DOE2 a), the blue light of the second corner point (P2) is guided fromthe in-coupling element (DOE1) to the out-coupling element (DOE3) viathe first expander element (DOE2 a), and the blue light of the secondcorner point (P2) is not guided from the in-coupling element (DOE1) tothe out-coupling element (DOE3) via the second expander element (DOE2b).
 5. The device (EPE1) according to claim 1, wherein the first guidedlight (B1 a) comprises light (B1 a _(P0)) which corresponds to a centerpoint (P0) of the input image (IMG0), the second guided light (B1 b)comprises light (B1 b _(P0)) which corresponds to the center point (P0)of the input image (IMG0), the third guided light (B2 a) comprises light(B2 a _(P0)) which corresponds to a center point (P0) of the input image(IMG0), the fourth guided light (B2 b) comprises light (B2 b _(P0))which corresponds to the center point (P0) of the input image (IMG0),wherein the out-coupling element (DOE3) is arranged to: form a firstoutput light beam (OB3 a) by diffracting light, which corresponds to thecenter point (P0) of the input image (IMG0), form a second output lightbeam (OB3 b) by diffracting light, which corresponds to the center point(P0) of the input image (IMG0), wherein the first output light beam (OB3a) and the second output light beam (OB3 b) propagate in a direction (k0_(P0)), which corresponds to the center point (P0).
 6. The device (EPE1)according to claim 1, wherein the in-coupling element (DOE1) is arrangedto diffract the input light (IN1) such that the first guided light (B1a) comprises light of a center point (P0) of an input image (IMG0), andsuch that the second guided light (B1 b) comprises light of the centerpoint (P0), wherein the out-coupling element (DOE3) is arranged todiffract the third guided light (B2 a) received from the first expanderelement (DOE2 a) such that the first output light (OB3 a) compriseslight of the center point (P0), wherein the out-coupling element (DOE3)is arranged to diffract the fourth guided light (B2 b) received from thesecond expander element (DOE2 b) such that the second output light (OB3b) comprises light of the center point (P0), wherein the light of thecenter point (P0) in the first output light (OB3 a) propagates in anaxial direction (k3,P0), wherein the light of the center point (P0) inthe second output light (OB3 b) propagates in the same axial direction(k3,P0).
 7. The device (EPE1) of claim 6, wherein the light of thecenter point (P0) in the first guided light (B1 a) propagates in a firstdirection (k1 a _(P0)), wherein the light of the center point (P0) inthe second guided light (B1 b) propagates in a second direction (k1 b_(P0)), wherein the angle (γ_(AB)) between the first direction (k1 a_(P0)) and the second direction (k1 b _(P0)) is in the range of 60° to120°.
 8. The device (EPE1) of claim 6, wherein a first region (REG3 a)of the out-coupling element (DOE3) is arranged to out-couple light ofthe center point (P0) received from the first expander element (DOE2 a),a second region (REG3 b) of the out-coupling element (DOE3) is arrangedto out-couple light of the center point (P0) received from the secondexpander element (DOE2 b), wherein the first region (REG3 a) overlapsthe second region (REG3 a) such that the common overlapping area (COM1)of the first region (REG3 a) and the second region (REG3 b) is greaterthan 50% of the one-sided area of the out-coupling element (DOE3).
 9. Adisplay apparatus (500) comprising an optical engine (ENG1) to form aprimary image (IMG0) and to convert the primary image (IMG0) into aplurality of input light beams of the input light (IN1), the displayapparatus (500) comprising the device (EPE1) according to claim 1 toform light beams of output light (OUT1) by diffractively expanding theinput light beams of the input light (IN1).
 10. A method comprisingusing the optical device (EPE1) according to claim 1 to provide theoutput light (OUT1), providing the optical device (EPE1) which comprisesthe waveguide plate (SUB1), which in turn comprises the in-couplingelement (DOE1), the first expander element (DOE2 a), the second expanderelement (DOE2 b), the out-coupling element (DOE3), and the one or moreoptically isolating elements (ISO1), forming the first guided light (B1a) by diffracting the input light (IN1) from the first grating (G1 a) ofthe in-coupling element (DOE1) having the first grating period (d_(1a)),forming the second guided light (B1 b) by diffracting the input light(IN1) from the second grating (G1 b) of the in-coupling element (DOE1)having the second different grating period (d_(1b)), forming the thirdguided light (B2 a) by diffracting the first guided light (B1 a) fromthe grating (G2 a) of the first expander element (DOE2 a), forming thefourth guided light (B2 b) by diffracting the second guided light (B1 b)from the grating (G2 b) of the second expander element (DOE2 b), formingthe first output light (OB3 a) by diffracting the third guided light (B2a) from the first grating (G3 a) of the out-coupling element (DOE3),forming the second output light (OB3 b) by diffracting the fourth guidedlight (B2 b) from the second grating (G3 b) of the out-coupling element(DOE3), and providing the output light (OUT1) by combining the firstoutput light (OB3 a) with the second output light (OB3 b), the methodfurther comprising: optically isolating the first expander element (DOE2a) and the second expander element (DOE2 b) by the one or more opticallyisolating elements (ISO1), thereby preventing direct optical couplingbetween the first expander element (DOE2 a) and the second expanderelement (DOE2 b), wherein the one or more optically isolating elements(ISO1) are a black absorbing material.
 11. A method comprising using theoptical device (EPE1) according to claim 1 to display an image (VIMG1),providing the optical device (EPE1) which comprises the waveguide plate(SUB1), which in turn comprises the in-coupling element (DOE1), thefirst expander element (DOE2 a), the second expander element (DOE2 b),the out-coupling element (DOE3), and the one or more optically isolatingelements (ISO1), forming the first guided light (B1 a) by diffractingthe input light (IN1) from the first grating (G1 a) of the in-couplingelement (DOE1) having the first grating period (d_(1a)), forming thesecond guided light (B1 b) by diffracting the input light (IN1) from thesecond grating (G1 b) of the in-coupling element (DOE1) having thesecond different grating period (d_(1b)), forming the third guided light(B2 a) by diffracting the first guided light (B1 a) from the grating (G2a) of the first expander element (DOE2 a), forming the fourth guidedlight (B2 b) by diffracting the second guided light (B1 b) from thegrating (G2 b) of the second expander element (DOE2 b), forming thefirst output light (OB3 a) by diffracting the third guided light (B2 a)from the first grating (G3 a) of the out-coupling element (DOE3),forming the second output light (OB3 b) by diffracting the fourth guidedlight (B2 b) from the second grating (G3 b) of the out-coupling element(DOE3), forming the output light (OUT1) by combining the first outputlight (OB3 a) with the second output light (OB3 b), and displaying theimage (VIMG1) having image points which correspond to output light beamsof the output light (OUT1), the method further comprising: opticallyisolating the first expander element (DOE2 a) and the second expanderelement (DOE2 b) by the one or more optically isolating elements (ISO1),thereby preventing direct optical coupling between the first expanderelement (DOE2 a) and the second expander element (DOE2 b), wherein theone or more optically isolating elements (ISO1) are a black absorbingmaterial.